Radiant energy – Infrared-to-visible imaging – Including detector array
Reexamination Certificate
2000-06-19
2002-10-15
Hannaher, Constantine (Department: 2878)
Radiant energy
Infrared-to-visible imaging
Including detector array
C250S338100, C250S338300
Reexamination Certificate
active
06465784
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
present invention relates to two-dimensional infrared solid-state imaging elements employing a thermal type infrared detector for detecting and absorbing incident infrared radiation and converting the same into heat.
DISCUSSION OF THE INVENTION
A thermal type infrared detector is a device which temperature is raised upon irradiation of infrared radiation by absorbing the irradiated infrared radiation that further performs detection of temperature changes.
FIG. 11
is a bird's-eye view showing an example of an arrangement of a single pixel of two-dimensional infrared solid-state imaging elements employing a conventional thermal type infrared detector utilizing a thermal type thin-film which resistance value is changed depending on temperature.
In the drawing,
1
denotes a semiconductor substrate comprised of semiconductors of e.g. silicon,
10
an infrared detector section being disposed in a spaced relationship with respect to the semiconductor substrate
1
,
11
a thermal type thin-film,
21
,
22
supporting legs for lifting and holding the infrared detector section
10
above the silicon semiconductor substrate,
31
,
32
metallic wirings for supplying current to the thermal type thin-film,
40
a switching transistor for switching between ON and OFF of current running through the thermal type thin-film
11
and the metallic wirings
30
,
31
,
60
a control clock wire for controlling ON and OFF conditions of the switching transistor, and
70
a metallic reflecting film for forming an optical resonance structure with the detector section in order to increase the absorption of infrared radiation at the infrared detector section
10
.
FIG. 12
is a view showing a sectional arrangement along current paths of the structure of a pixel of the two-dimensional solid-state imaging elements employing a conventional thermal type infrared detector as shown in
FIG. 11
wherein the switch-transistor
40
, signal wire
50
and control clock wire
60
are omitted since these are not directly concerned in the present invention.
As already mentioned, the thermal type thin-film
11
is formed above the infrared detector section
10
wherein the metallic wiring
31
,
32
are connected to the thermal type thin-film
11
and further connected via contact portions
122
,
122
to a signal read out circuit (not shown) formed on the silicon semiconductor substrate.
The thermal type thin-film
11
and metallic wiring
31
,
32
are covered by insulating films
100
,
110
of silicon dioxide film or silicon nitride film wherein these insulating films
100
,
110
constitute the mechanical structure of the infrared detector section
10
and supporting legs
21
,
22
.
80
denotes an insulating film for insulating the signal read out circuit and the wiring
31
,
32
that are formed on the semiconductor substrate
1
, and the light detector section
10
is disposed above the metallic reflecting film
70
above the insulating film
80
with a hollow section
90
being interposed therebetween. Another insulating film may be formed on the surface of the metallic reflecting film
70
.
Next, operations of conventional two-dimensional solid-state imaging elements employing such a thermal type infrared detector will be explained.
Infrared radiation is made incident from a side at which the light detector section
10
is disposed and is absorbed by the light detector section
10
.
Owing to the presence of the metallic reflecting film
70
, stationary waves of incident infrared radiation wherein the position of the metallic reflecting film
70
forms a node are formed, and by suitably setting the distance between the infrared detector section
10
and the metallic reflecting film
70
, absorption of infrared energy can be increased in the infrared detector section
10
.
Infrared energy that has been absorbed at the infrared detector section
10
is converted into heat and increases the temperature of the infrared detector section
10
. The degree of temperature rise is dependent on the amount of incident infrared radiation (while the amount of incident infrared radiation is dependent on the temperature and thermal emissivity of an object to be picked up).
Since the degree of temperature rise can be known by measuring a change in resistance values of the thermal type thin-film
11
, the amount of infrared radiation that is emitted by the object to be picked up can be known from changes in resistance values of the thermal type thin-film
11
.
As a material for the bolometer that exhibits large changes in resistance owing to changes in temperature, semiconductors of vanadium oxide (VOx) or the like may be employed as known from reference P. W. Krise, “Uncooled IR Focal Plane Arrays”, Proceedings of SPIE, vol. 2552, pp. 556-563.
In case resistance temperature coefficients of thermal type thin-films
11
are identical, the larger the temperature rise of the infrared detector section
10
is, the larger the change in resistance that is obtained by an identical amount of incident infrared radiation becomes, and the higher the sensibility becomes. In order to increase the degree of temperature rise, it is effective to reduce the amount of heat escaping from the infrared detector section
10
to the silicon semiconductor substrate
1
as little as possible, and due to this fact the supporting legs
21
,
22
are designed as to limit thermal resistance as much as possible.
It is also important to set a thermal capacity of the infrared detector section
10
small such that a temperature time constant of the infrared detector section
10
becomes smaller than a frame time of the imaging elements.
While infrared radiation is made incident into entire pixels, only those that are made incident into a portion of the infrared detector section
10
contribute to the temperature rise of the infrared detector portion
10
(although some amount of infrared radiation that is made incident into the supporting legs which are close to the infrared detector section
10
are also effective), and infrared radiation that is made incident into remaining regions become ineffective.
Due to this fact, it can be easily understood that it is also effective to increase an aperture ratio (a ratio of an area of the infrared detector section
10
with respect to an area of the pixel) for increasing the sensitivity.
In a method for detecting changes in temperature by using a borometer as explained above based on a conventional example, it is necessary to employ a material of large change in resistance caused by temperature and low noise such as vanadium oxide (VOx) that is usually not used in a silicon process.
While such a material can be treated in film-forming, photolithograpy or etching processes using similar manufacturing techniques as known for silicon processes, it has been difficult to perform processes in manufacturing lines that are used for silicon VLSI in view of contamination of silicon processes.
Further, in the arrangement of the conventionally known infrared solid-state imaging device as shown in FIG.
11
and
FIG. 12
, the infrared detector section
10
needs to be formed at most on a region other than the supporting legs
21
,
22
and contact portions for connecting these supporting legs and the read out circuit that is formed on the silicon semiconductor substrate
1
, whereby the aperture ratio was restricted by the design of the supporting legs, contact portions and interval clearance between these portions and the infrared detector section
10
such that high sensitivity could not be obtained.
Such problems became more remarkable the smaller the pixels were so that it was difficult to obtain high resolution using small pixels while maintaining proper sensitivity.
The present invention has been made in view of the above problems, and it is a purpose of the present invention to provide infrared solid-state imaging elements which are two-dimensional infrared solid-state imaging elements that form a thermal type infrared detector on a same semiconductor substrate as
Gagliardi Albert
Hannaher Constantine
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